There is a 100nF capacitor in the small high-voltage power supply on the display board. If it fails, the 60V line fails to regulate properly and either you get low, or unstable voltage. I am not honestly sure which component this is, or where.

Alternatively, if there is an internal failure in the VFD, then you can also see low voltage at the test point TP1 on the display.

If the coil breaks a contact, the circuit fails to work completely.

If one of the two electrolytic capacitors on the back of the display fails, then you can also get low voltage.