Test case is the following:
var mvtSource = new L.TileLayer.MVTSource({...})
map.addLayer(mvtSource)
map.removeLayer(mvtSource)
This code results in a JS error in L.TileLayer.bringToFront() method.
Here is online demo: http://aparshin.ru/temp/mvt_bug.html
Looks like the reason is that you are trying to do some actions when previously requested vector tile is received for already removed layer.