Now here's a rather worrying bug I stumbled upon last night.
There's something strange going on at the beginning of automation items: the envelope value fades in from the value at the end of the last automation item, (or if you don't bypass the underlying envelope, from the envelope's last value) to the actual value set by the envelope. This fade seems to be a constant 552 samples long and affects everything you can control with an envelope, including JS and VST parameters, so I'd say this is quite severe. I can't test AU as I don't have a Mac but I assume this is an universal AI bug.
Tooltips also display the actual value of the envelope, not the displayed one.
I've attached a small project to illustrate the problem. Try moving/adding envelope points between markers 1 and 2 (the initial 552 samples of the AIs), and also around marker 2, and strange things (or nothing where something should) happen, especially on the first 3 tracks with square shape envelope points. I've used 1kHz sine waves and pre-fx volume envelopes to better visualize this. The master is muted for a reason, you're really not supposed to play this (or at least not more than 1 track at a time) as the sine waves peak at 0db and you definitely don't want to play them all at once. You've been warned
REAPER v5.965 x64, Win10 Pro x64 v1809, SWS v2.10.0 #1 (if that matters since there's another recent AI/SWS bug report)