Preferences > Editing Behavior > Automation > "When adding volume/pan envelope, apply trim to envelope and reset trim:"
I have that option set to "Never", although often times after installing a newer version of Reaper 5 (Portable and Normal) this setting gets changed to "Always"!
I've seen this happen multiple times now, from v5.18(my first usage) to v5.50_RC22 x64 on Win7_x64
Thanks.