Yep, I added it to my collection and it certainly doesn't apply +1 to spell attacks. I see that you used the modifiers Bonus - Melee Spell Attacks and Bonus - Ranged Spell Attacks. The official items that add a bonus to spell attacks use Bonus - Spell Attacks; there aren't any that use the Ranged/Melee versions, and it's possible that those modifiers don't work for homebrew magic items (but might work for class features.)
I've tested it out, logged out and back in, and waited for the item to update. The +1 never gets applied to my attack spells.
Item: https://www.dndbeyond.com/magic-items/3902849-shield-of-1-spell-attack
Character: https://www.dndbeyond.com/profile/BlueDwarves/characters/57989859
Yep, I added it to my collection and it certainly doesn't apply +1 to spell attacks. I see that you used the modifiers Bonus - Melee Spell Attacks and Bonus - Ranged Spell Attacks. The official items that add a bonus to spell attacks use Bonus - Spell Attacks; there aren't any that use the Ranged/Melee versions, and it's possible that those modifiers don't work for homebrew magic items (but might work for class features.)
Helpful rewriter of Japanese->English translation and delver into software codebases (she/e/they)