Panaracer's purple TPU tube,
Purple Light,

After installing on both front and rear wheels,
I've had multiple cases where there was a difference in air loss rates.
TPU tubes have naturally gradual air loss, so
for example, from 6 bar pressure,
you wouldn't lose 2 bar in just one day (←this actually happened).

After investigating, the fast air loss turned out to be
caused by loose valve cores.
Since the valve cap is aluminum,
when removing it by turning,
the valve core that was already loosely threaded
gets loosened further due to co-rotation—this seems to be a pattern.
If you notice a difference in air loss rates between front and rear wheels, or
the air loss seems faster than typical for a TPU tube,
I recommend checking this.