TMR1 - strange behaviour when toggeling a BIT variable on overflow

[Acetronics2]

RTFM Datasheet § 6.5.1 ...

For writes, it is recommended that the user simply stop
the timer and write the desired values. A write
contention may occur by writing to the timer registers,
while the register is incrementing. This may produce an
unpredictable value in the timer register.

Might be a good reason ...

Alain