I am having a similar problem (just upgraded from Infragistics 10.3 to 11.1 - issue did not occur in 10.3 btw), but disabling EnableMultiDayEventBanner is not an option (my customer wants multi-day appointments to span as a solid bar if that makes sense -- disabling this setting just cuts the event up into separate "pieces"/days that don't line up.)

Looking at the html my aspx rendered for the webmonthview, I see the calendar is a 8-column html table and appointments are rendered as table rows.  Multi-day appointments achieve that solid bar look by using a colspan attribute which makes sense.  But the problem with the icontrol is that it will add appointments to a table row that will result in a total column count greater than 8.  For example, in one  instance I had  appointments that spanned 3-day, 5-day, 2-day and 1-day all added to the same row which resulted in 12 columns (counting the left hand gray number column) which caused the table to  shrink and the table row to expand out to the right (see posted picture).  Two of the appointments had the same start date of July 11th (the 3-day and 5-day appointments) -- the 3-day appointment rendered ok but the 5-day appointment rendered after the 3 day (started on July 14th instead of July 11th).