Posting this as a top level comment as well for visibility.
The tablets are a scam.
Ridiculously detailed video about them here.
The TL;DW is: use powder over tablets, and fill both the main detergent section AND the prewash section (or just throw a little extra powder in loose)
Also, run the hot water before turning it on
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Yep, the video (or possibly the follow up video? don’t remember) goes into this. I’m in a very hard water area, so we fill it, but you’re right, it should depend on hardness.
Which incidentally, is another reason tablets suck.