Premiere Pro 15.2 caption issue

I'm using subtitles, yes, created from a new captions track and pasting text in paragraph by paragraph from a Word doc. I'm splitting each paragraph into smaller segments, then adding a new segment (+) for each paragraph. Some of the subtitles butt together, some have a second or two of space, but that makes no difference. There's only a single video layer - video file is around 90mins long (2.5Gb mp4), but edited down to around 10mins of highlights.

Quick test: I started a new project, imported a 45 second video clip and created some dummy subtitles. A mixture of new segments, split segments and copy-pasted segments. Short phrases. Everything's fine until I get to around 10 segments, when the problem recurs.

Behaviour:

1. When I click on the penultimate caption in the Text panel, the final caption is highlighted. It takes about 10 seconds or so of clicking other captions in the panel before I get any response. Then I can freely edit any caption apart from the last two.

2. When I click on the final caption in the Text panel, the final caption itself is highlighted, but I can't edit it or click off it for a variable amount of time.

3. Sometimes I can double click the final panel from cold (i.e. instead of clicking it once to highlight it first) and go into edit mode, sometimes I can't. It's random.