Hi, I never had such problem before with LRT5.
I made an edit of a timelapse sequence (a nice sunrise, from full night with moon light, with Tonga thin clouds in stratosphere producing nice colours).

I edit with good exposure in lightroom (then of course choose "save metadata to files" normally), then back in LRT, autotransition.
And the preview look massively overexposed, with also holygrail not working (images below)

Maybe I did something wrong while exporting from LRT to Lightroom ?
I did as before with LRT5

Are you on Lightroom 11? If you are on an older Lightroom, you'd need to set the masks version to 1.0 in the settings and start over via "Metadata / Initialize".

If you are on LR 11, ok - but the way you exposed that sequence is not really even. Look at the blue curve in the third screenshot, it's going up and down. The orange compensation curve has to do a lot of work to compensate that. The orange curve is brightness adjustments. Try to get that curve closer to the horizontal middle line via the two sliders. The closer it is, the less adjustments LRTimelapse needs to make.
Also I'd recommend to set a reference area at the beginning (to the sky). This might also help.
Please watch my Holy Grail Tutorial: https://lrtimelapse.com/tutorial/

But in any case, which such an uneven exposing of the original sequence, you'd always get changes in the luminosity in Lightroom, that is the leveling that LRTimelapse does. Then with editing the keyframes you can correct this via Exposure.

I'm sure you'll understand everything better if you watch my tutorials.