See the commit a00c9d5 (#12926) for a detailed description of the problem, a regression introduced in 26d5032 (#12846).

Same approach here as the linked fix for rp2 (applied unconditionally here as this port only supports USB-CDC for stdin/stdout).

For test code I used the following:

from machine import Timer
import time


def print_lots(_):
    print(b"ABC" * 120)


t = Timer()
t.init(mode=Timer.ONE_SHOT, period=1000, callback=print_lots)
time.sleep(2)
print("Done!")

Only prints b and then hangs due to the regression, works again with this fix applied (prints the long bytes object, and then prints "Done!".)