If you are sprinting and use your shield, there will be sprinting particles at the feet. This only occurs directly from a sprint. If you shield THEN try and sprint, then the problem will not occur.
I don't know why, but it always errors when I try uploading photos, so here is a link instead (photo WITHOUT the F3 screen up is in 1.16.4 and other photo is 20w51a)
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Duplicate of MC-152728