So first off, the bug has been reported in the bug forums (remains to be seen if SE will give this the top priority it needs, cause it's game breaking for the bard). The bug occurs when the BRD uses Apex Arrow which imparts the Frontline March buff on the BRD and nearby allies. If the BRD then uses Powerful Shot while this buff is up, he will permanently lock into walking speed, only. Even after the buff expires, the BRD remains at walking speed. Even worse, the lock will continue through death and respawn. Firing Powerful Shot again after the buff has expired will restore normal movement speed, but this is not very convenient, as it requires a target. Also, the problem will return upon firing Apex Arrow followed by Powerful Shot while the buff is up.
There are two methods to work-around the bug that I have learned about, tested, and found to be the best so far for quickly restoring normal movement speed in a pinch:
1. Initiate mount summon. The summon does not have to complete; it just needs to be that momentary instant of pressing the button to start the summon. This only works in Frontlines PvP.
2. Initiate use of the Standard-Issue Elixir. Again, you don't have to complete the cast for it. Just press the button to initiate it.
Both methods do work, according to my testing; however, I must caveat that with the fact I am playing on PS4 Pro. I do not know if these methods will work on PC (for that matter, as I understand, it's mostly PS4/PS4 Pro players are affected by the bug). It must be noted, however, that if the BRD uses Powerful Shot again while Frontline March is still up, he will be locked to walking speed again and will need to use one of the above methods to reset normal movement speed.
Hopefully, this helps people deal with this issue to make their bards more playable/viable in PvP until SE finally fixes the problem.
EDIT: This was finally fixed in patch 6.11.