version 3.12...
Choose any damaged unit in Repair Bay... set Salvage... choose any location... the units Engine appears as a Salvageable part.
The only item of note is that I am playing on a 0.61 G world. No errors in log file. Error occurs with GM Edit Damaged units as well which probably rules out Gravity.
https://sourceforge.net/p/mekhq/bugs/814/
Reported this a while back as well.
To work around this issue you need to save and reload. However if you do... Shoulder/Hip hits disappear.
Duplicate of Bug [#814].
Related
Bugs:
#814