Okay so after testing a little bit more there seems to be a similar although not exact bug with Rotate and Scale as is with Size and Position and Text: Simple which I noted as #2 in my V19.08 beta report: https://forum.shotcut.org/t/v19-08-beta-now-available/12358/3
The situation with Rotate and Scale is that if you have keyframes in a clip then decide to extend that clip, even if you add more keyframes in that newly extended section those keyframes will not be acknowledged. When this is done with Size and Position/Text: Simple, the object just disappears in the newly extended section with keyframes but with Rotate and Scale (and possibly other filters) simply nothing will happen.