Fun fact: the iOS bug where you could brick a device by sending some strings in Arabic to the device was essentially due to assuming that there is a monotonic relationship between some measure of "string length" and "display space".
(essentially, iOS was trying to truncate strings in notifications to make them fit available space, except it did that in a very naive way, and would crash on any case where removing codepoints from a string made it display _longer_. Pretty easy to trigger with letters like ي)
The same letter can look different depending on whether it's at the beginning, in the middle of, or at the end of a word. So if you remove a letter with a fairly short final form, which is preceeded by a letter with a fairly short medial form but a fairly long final form, it's possible that the new rendering of this letter would be longer than the previous two.
This problem isn't exactly Arabic-specific, it's just super easy to trigger in Arabic. You've got kerning in the Latin script as well, and you could theoretically have a font that makes `the` look smaller than `th`.
But in Arabic, letters change form based on where they are in the word. All three things with two dots on them are the same letter in تتت, but the middle one is much smaller. I can imagine something like يم being truncated to ي and the end result being longer visually, depending on the font.
(essentially, iOS was trying to truncate strings in notifications to make them fit available space, except it did that in a very naive way, and would crash on any case where removing codepoints from a string made it display _longer_. Pretty easy to trigger with letters like ي)