This can happen during pre-roll when buffers are filled
to start audible playback at zero.
While the position argument is signed for all plugin-standards,
it seems that some do not support negative timestamps before
00:00:00:00. (e.g. https://github.com/falkTX/Carla/issues/1236)
Furthermore TempoMap::bbt_at_sample() returns 0 for all negative
timestamps, but it was possible tthat tempo-map transmission,
as well as beat-position returned negative values.
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