That nose-dive type malfunction is almost guaranteed to be a mag issue, in my experience.
Glock OEM reduced capacity 10round mags are not known for reliability and the different follower can cause issues.
Maybe try replacing the springs with Wolff +10% springs or try Magpul 10round G19 mags. Remember, the Glock relies on the mag spring as part of the extraction/ejection cycle. If the mag springs are weak, you can have a slow down in the ejection process which can lead to feeding issues.
I’d also try shooting the 26 with standard OEM G26 mags (add a Pearce +0 Pinky shelf type mag floor-plate to get a full-size grip feel) and see what shakes.