With a little "wait_ms()" its more clear what you mean :-)

from m5stack import *
from m5ui import *
from uiflow import *

setScreenColor(0x111111)
label1 = M5TextBox(05, 05, "a", lcd.FONT_Default,0xFFFFFF, rotate=0)
label2 = M5TextBox(05, 20, "x", lcd.FONT_Default,0xFFFFFF, rotate=0)
label0 = M5TextBox(16, 50, "A", lcd.FONT_DejaVu72,0xFFFFFF, rotate=0)

for angle in range(91):
  label0.setRotate(angle)
  label1.setText('angle:' + str(angle))
  wait_ms(50)

for posX in range(16, 256):
  label0.setPosition(x=posX)
  label2.setText('pos-x:' + str(posX))
  wait_ms(100)

I am sure it is ones more a bug :-)
You can check this, if you let the angle zero. You can see that the char disappears at pos-x = 33. And the same behavior occurs, if you have rotated the char. It is obviously one more of the uncountable bugs.

@M5Stack: Why does the M5Stack not draw a char if a small part is outside of the screen? We can display a char half way, if it is rotated, but not if it is at angle 0 degrees. That makes no sense. Maybe someone would like to program a scrolling text so that chars appears on the right and disappears on the left.

Best regards
Thomas

from m5stack import *
from m5ui import *
from uiflow import *

setScreenColor(0x111111)
label0 = M5TextBox(16, 38, "A", lcd.FONT_DejaVu72,0xFFFFFF, rotate=0)
angle = None
x = None
angle = 0
for count in range(90):
  label0.setRotate(angle)
  angle = (angle if isinstance(angle, int) else 0) + 1
x = 16
for count2 in range(25):
  label0.setPosition(x=x)
  x = (x if isinstance(x, int) else 0) + 1

And the video:
https://youtu.be/qsKmaJEeI6A

The "problem" in your example is, that the letter D is to wide to display completely on the display.
Unfortunately letters are suppressed if they have not enough space on screen.

This Example works :-)

from m5stack import *
from m5ui import *
from uiflow import *

@timerSch.event('timer1')

def ttimer1():
  
  global curLetter

  if curLetter == "A":
    curLetter = "B"
  else:
    if curLetter == "B":
      curLetter = "C"
    else:
      if curLetter == "C":
        curLetter = "D"
      else:
        if curLetter == "D":
          curLetter = "A"
          
  label0.setText(curLetter)
  label1.setText(curLetter)
  pass

setScreenColor(0x000000)
curLetter = "A"
label0 = M5TextBox(25, 20, curLetter, lcd.FONT_DejaVu72,0xFFFFFF, rotate=0)
label1 = M5TextBox(25, 80, curLetter, lcd.FONT_DejaVu72,0xFFFFFF, rotate=0)

timerSch.run('timer1', 500, 0x00)

Best regards
Thomas

https://www.youtube.com/watch?v=nYwlS4ADqOg&feature=youtu.be

I tried firmware versions 1.4.4, 1.4.5, and 1.4.5.1 thinking maybe that's related but they all behaved the same.

I'm not really sure what else I can do to troubleshoot this since it seems unique to my device.