My tube lose air once inside Tyre and mounted on wheel?
Check the rim for protrusions - is the rim tape misaligned?  Are there any lumps/bumps?  Any spoke ends prodruding just a bit too far?

Check the inside of the tyre/tire for lumps and bumps too.

Are you using a presta valve in a schrader-sized rim hole?  Could be opening a small hole there when pressurising.

Do you always put the tube in the same way around?   Try flipping it to move any weak point around.

Hmm - "weak point"  raises a thought...  Perhaps you should cut your losses and just fit a new tube.  They're consumable; I guess your current one has done its dash.   Don't want the weak point to turn into a blowout.