The bug
Much like when the colored beacon beam was first introduced, placing a stained glass block above a beacon beam does not change the color instantly anymore. It takes multiple seconds to update the beam color.
Related issues
Attachments
Comments
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kumasasa
MC-121105 describes the same issue in 1.12.2 but only when the player is in a certain distance to the beacon
qwerty23495
Duplicate of MC-127238.