Just changed the code as below:
Timer1.stop();
TIFR1 |= (1 << TOV1); //clear all Timer 1 Interrupt Flags
Got this from the data sheet:
• Bit 0 – TOV1: Timer/Counter1, Overflow Flag
The setting of this flag is dependent of the WGM13:0 bits setting. In Normal and CTC modes, the TOV1 Flag is
set when the timer overflows. Refer to Table 16-4 on page 132 for the TOV1 Flag behavior when using another
WGM13:0 bit setting.
TOV1 is automatically cleared when the Timer/Counter1 Overflow Interrupt Vector is executed. Alternatively,
TOV1 can be cleared by writing a logic one to its bit location.
It seems to me this solved the problem.