It's a known problem, It's because the chipset on your TV can't handle properly the process of embedding the subtitles into the images (i.e. transcoding in realtime).
Try closing background running apps, it's a bit better for me.
On my Android 8.0 TV, I get the error "There was a problem parsing the package" when trying to install VLC 3.4.0 Beta 4 and Beta 5, whereas Beta 3 works fine. I tried from different repositories with the compatible architecture (armeabi-v7a) but still no chance. I'll probably wait for anot...
To display subtitles, VLC has to transcode every key frames (for example when the background changes) the video to embed the text lines, while also decoding the video itself. Sony's SOC are reputedly too weak to perform this flawlessly. Disabling or force-closing useless background apps (Google Musi...
Any update on this ? I don't even use a Firestick but I have the same problem here, some anamorphic movies are forced in 16/9 no matter switching the aspect ratios.
Sorry to dig up this topic, but I still encounter this problem, whatever the video format (mkv, mp4...), I'm on W10 and use SMBv1 (SMBv2 and V3 don't work with VLC).