ALT CRZ after Step Climb - Persistent "White Box" FMA

[Sabretooth78 56]

After executing a step climb on my latest flight, the white box around the ALT CRZ annunciation persisted for about 35 minutes, and counting.  I performed the step climb as I always have in the A320 - dial new altitude in the FCU, enter VS (to smooth the transition to climb) and then pull altitude for open climb.  I also pushed the altitude knob and entered "400" into the Cruise Perf for good measure after reaching the new cruise level, but that had no effect in eliminating the box.  Altimeters are also in agreement, etc.

 

Just before step climb:  (note no box while at original ALT CRZ)

 

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Some time after step climb (@ YQT):

 

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Almost to YWB (Winnipeg):

 

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...and still going strong 6 minutes later.

 

EDIT:  I stepped away, but sometime before YDR it finally extinguished.  Still, lasted at least an hour.

 

Flight plan, if interested:  CYYZ F380 AVSEP5 VIGLO DCT SSM DCT YQT/F400 DCT GERTY DCT YWG DCT YDR DCT OTPUS DCT 51N115W DCT BOOTH CANUC4 CYVR