Hi, @freyalupen!
Could you please check this new package? (or just update master to 6d87f89408cec4)
krita-5.2.0-prealpha-line-issue-dk2.zip — Яндекс.Диск
I have fixed a race condition in KisAsyncMerger::setupProjection that you pointed out. Tough I could barely reproduce it (even with custom msleep(10) calls and asserts), so I do need a confirmation that my fix actually fixes the issue 