... the D4i doesn't even have a gauge mode that you can select.
The D4i manual (and presumably the instrument itself) calls this "Free mode". It seems to be functionally equivalent to other Suunto models' (e.g. Cobra) "Gauge mode", including the automatic reversion to this mode for violating a mandatory Deco stop. So read these as synonyms and maybe we'll make some progress.
Perhaps I jumped to conclusions; it is
possible to accidentally press the series of buttons to set the D4i into free mode, where it will behave pretty much as described, including the 48 hour no-fly, no plan mode, can't change to dive mode, etc. I'm not personally familiar with the D4i to know if the display stays at "Er" when you blow the mandatory stop, or that goes away. The manual pages 41 and 79 suggest that the "Er" display might be temporary, but it's not really clear.
So if you just accidentally set it to "Free mode", my apologies. But before dropping the possibility that "Er" mode was involved, consider the information on page 65:
5.10.2. Mandatory Safety Stops
When the ascent rate exceeds 10 m/33 ft per minute continuously for more than five (5) seconds, the microbubble build-up is predicted to be more than is allowed for in the decompression model. The Suunto RGBM calculation model responds to this by adding a Mandatory Safety Stop to the dive. The time of this Mandatory Safety Stop depends on the severity of the ascent rate excess.
So it doesn't take a "table depth/time violation". as we commonly think of it, to require a mandatory stop, just ascent rate will do it. Your profile shows an 8 m ascent in 39 seconds, if I've interpreted it correctly. That's 12.3 m per minute for 39 seconds, sounds like it meets the spec.
Accidentally setting "Free mode" seems unlikely enough to want to consider other possibilities. But it could be the explanation.