Jeremy Burge at Emojipedia explains what’s happening when iOS 11.1 replaces the letter ‘I’ with an ‘A’ followed by ‘⍰’:

What’s really going on is that the letter “I” is being appended with an invisible character known as Variation Selector 16 when auto-correct kicks in.
This VS-16 character is intended to be used to make the previous character have emoji appearance.¹ When used in conjunction with the letter “I” it displays in some apps as “A ⍰”.

The correct behaviour should be to ignore the invisible variation selector if the previous character doesn’t have an emoji version.

The bug, which was a hot topic on Twitter over the weekend, only affects some users. Until a software update is issued by Apple to fix the issue, the company recommends setting up a text replacement rule that replaces a capital ‘I’ with a lower case ‘i’ as a workaround.