Rigidbody acting strangely when "is kinematic" is unticked

I’ve seen numerous questions and threads about rigidbodies behaving strangely, but for me it’s linked to whether they had “is kinematic” ticked from the outset or not.

In my case, objects with rigidbodys that aren’t kinematic - that is, they are affected by physics - at game start are generally fine for me. However, if I have an object that I first use as “is kinematic” and then untick this box in a gameplay event, so that its physics is turned on, so to speak, that object will behave much more erratically, it will seem far too light no matter how much mass I give it, and will bounce and twitch when I bump into it with the character.

Has anyone experienced anything similar, and can share any ideas?

Could be that it has strange settings for center of mass or inertial tensor, unintended collider layout? Hard to really guess at much that is specific without any info on how you have this Rigidbody set up/what else you are doing with it.