turning switch off to ceiling fan/light does not immediately kill the light

This issue just started occurring in the last week or two. The fixture has been installed for ~18 mo with no issue.

The light gets brighter momentarily–maybe a 1/4 of a second then goes off very briefly, then lights again for a 1/2 sec, before finally going and stay off.

The bulbs are incandescent. The switch is a plain SPST–no dimmer, or ‘smarts’

Ideas on cause?