Can you say exactly what steps you were doing when the deletion happened. I cannot reproduce that and no one else has thrown in a “me too” yet.
By default the Gallery starts in Timeline mode. From that mode I viewed a number of images, swiping up and down among them. The only thing happening is my swiping mostly being interpreted as a side swipe. No deletions at all.
Given that the gallery app is essentially the same as the one included in a majority of custom ROMs (package name com.android.gallery3d) the behavior should be the same across the board. I have never seen or heard of deleting when swiping up/down.
Was curious so I checked/compared on other ROMs I run.
/e/OS 0.9 Pie, Gallery (com.android.gallery3d), v1.1.40030.
Candy7 Nougat, Snapdragon Gallery (org.codeaurora.gallery), v1.1.40030.
AOSiP Oreo, Gallery (com.android.gallery3d), v1.1.40030.
crDroidAndroid Nougat, Gallery (com.android.gallery3d), v1.1.40030.
/e/OS 0.9 Nougat, Gallery (com.android.gallery3d), v1.1.40030.
[ Gallery has essentially gone unchanged, hence the same version number through three Androids ]
Did a lot of swiping. No deletions. I just can’t see where there’s a UX problem. Don’t forget, /e/ didn’t create the Gallery app.
I’ve never used the film-strip view before, in all these years, so I tried that. From there one can swipe up/down to delete files. You can undo each swipe also ( Heh, my thumb was obscuring the Undo button I didn’t know was there. So I permanently “lost” a few test images).
Anyhoo, since you said “timeline” and not film strip, I’m not sure what’s what.