Bug?: Cursor moves after turning on SMCP

When I switch to smallcaps, the cursor moves a few places to the left, but has effect on its original position. I sent a video to the support email for clarification. I tried restarting Glyphs, didn’t help.

Update: the behavior stops when I change the smcp feature.

It had a line in it that said:
sub germandbls by s.sc s.sc;

Problem is fixed if I change it into:
sub germandbls by germandbls.sc;

But, that’s not what I want. So how do I make option 1 work normally?

Many thanks in advance!

Glyphs loses track of cursor position when the original number of glyphs changes after substitution (it’s more apparent in liga feature and such, which result in fewer glyphs in general). Other apps will behave differently, so test the behaviour there.