I’m not from Google, not even an expert in DevTools, just pass by after reading a few lines of code in Chrome DevTools.
There are a few reasons why a frame is dropped, you can check at FrameDropReason.
But to understand what is the reason for the dropped frame, I don’t find any better way other than reading the profiling JSON, which you may find something like this:
{"args":{"data":{"compositeFailed":8192,"unsupportedProperties":["background-color"]}},"cat":"blink.animations,devtools.timeline,benchmark,rail","id2":{"local":"0x3b08a0fb80"},"name":"Animation","ph":"n","pid":23024,"scope":"blink.animations,devtools.timeline,benchmark,rail","tid":259,"ts":71014192568},
Basically says the frame dropped by the compositor, because of unsupported CSS probs. More details on Compositor Failure reasons here.
It’s also good to look into this issue which tracks the addition of Dropped Frame.
Again, I’m not entirely sure, just some hints for you to look into.