SAN DIEGO -- The growth of a young team is not linear. It often takes the well-traveled path of one step forward, two steps back.

Tyler Soderstrom took a big step forward on Tuesday night at Petco Park. The A’s however, took those dreaded steps back with a 4-3 loss on Kyle Higashioka’s walk-off home run in the ninth inning. That ensured a sixth straight series loss for the A’s, who also have lost four straight games.

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Before Higashioka connected on Scott Alexander’s changeup, Soderstrom had the biggest hit of the evening. He tied the game in the eighth inning with a two-run homer off right-hander Enyel De Los Santos.

It was Soderstrom’s second homer in as many games.

“Tyler’s at-bats have gotten better and better,” A’s manager Mark Kotsay said. “He went through a tough stretch. At some point, he just kind of found a rhythm, a groove. The at-bats have been very good of late. That was a huge homer for us.”

It was just last week that Soderstrom was in a rut. The 2020 first-round Draft pick had gone hitless from May 21 to June 5, spanning 16 at-bats. During that stretch, Soderstrom rotated at first base with J.D. Davis and Aledmys Díaz.

Soderstrom has been in the lineup each of the past six games, however, and he has gone 6-for-17 with the two homers. Tuesday marked his third multihit game of the season but his first since May 13.

“I was going through a rough spell,” Soderstrom said. “I just put in the work every day, made a couple adjustments, and it’s been paying off.”

The A’s were down, 3-1, when Soderstrom stepped in against De Los Santos with Miguel Andujar on first base. The left-handed hitter went down 0-2 in the count, then laid off a changeup in the dirt. De Los Santos left a fastball over the heart of the plate, and Soderstrom pounded it to center field, a Statcast-projected 421 feet with 106.9 mph exit velocity.

“He threw me a slider and two changeups, so I was looking for something up and over,” Soderstrom said. “I thought he was probably going to throw me a heater there, so you try to be on top of it. It worked out in my favor.”

Soderstrom’s 69 homers in 295 Minor League games speak to his power potential. Ranked by MLB Pipeline as the A’s No. 1 prospect as recently as last year, he rated a 60 for power on the 20-80 scouting scale.

Consistency at the plate has yet to come at the Major League level, however. Soderstrom carries a .181 career batting average through his first 188 at-bats. Perhaps regular playing time will prompt consistency; perhaps consistency must be shown to earn regular playing time.

The A’s coaching staff, front office and, yes, his teammates all know the potential remains for Soderstrom to become a middle-of-the-order stalwart.

“It’s awesome he put the bat on the ball like that,” A’s starting pitcher JP Sears said. “I feel like every time he goes to the plate that can happen. He’s showing a lot more confidence, really good at-bats and good swings. …

“There’s a reason he had such good hype in the Minor Leagues, such good power.”

As deflating as a walk-off defeat can be, the A’s owned up to the fact the Padres’ first three runs were avoidable. Sears was working on a three-hit shutout through 4 2/3 innings before he hesitated on the mound and didn’t cover first base in time to prevent an infield single by Luis Arraez.

Not only did Ha-Seong Kim score on the play to tie the game, 1-1, but it extended the inning. Sears allowed hits on four straight pitches (Arraez, Fernando Tatis Jr., Jurickson Profar and Manny Machado) as the shutout turned into a 3-1 deficit through five frames.

“He froze,” Kotsay said of Sears’ late break. “He talked about it coming off [the field]. There was a lot of emotion after that inning, a lot of frustration on his end. … He’s never froze before; he’s always covered the bag.”

Said Sears: “I’ll take ownership of it. It’s one of those moments where you’re really mad at yourself. I’ve got to do a better job of getting past that one play and make a pitch to the next batter.”