Apparent display bug with Arabic letter sequence lam meem alef

I've noticed that on Mac, in any of three Web browsers (Chrome, Safari, Firefox), the sequence lam mim alef (لما) is sometimes displayed incorrectly. It should look like this:

But instead it looks like this:

(Ignore the black/white color flip — I had to take screenshots in different contexts.)

Notice,

• The mim is completely gone!

• The lam has the "hat" on it that's normally associated with the initial form. This is expected, because it's in the initial position.

• But it also has a tail sticking out to the right, which is not normal for an initial form.

By contrast, the following letter sequences seem to be displayed correctly:

(lam+mim, and mim+alif).

And the "compulsory" ligature lam+alif is displayed correctly:

Where the sequence is displayed incorrectly:

• In some web pages, such as this one (look for آلمان / Germany)
• In the URL bar (and status bar) of Chrome, Safari, and Firefox (try this Forvo URL).

Where the sequence is displayed correctly:

• On some web pages (e.g. Forvo or Google Translate)
• In LibreOffice
• On Windows (I tried the above "where it doesn't work correctly" places)

Since it works in some web pages and not others, and not in the URL bar, does that mean it's likely to be a font problem?

When I copy and paste the characters from a place where they display correctly to a place where they display incorrectly, they display incorrectly. I.e. they're the same characters, but the display is different depending on where they're shown. (And the reverse is also true.)

I happen to be using these letters in a Persian context, but they're Arabic letters and I believe the issues are the same.

I'm not an expert on either Arabic or Persian, so there could be something I'm missing… please let me know if I'm misunderstanding the issue. However I've searched the information I could find on Arabic and Persian ligatures, and there doesn't seem to be anything corresponding to this display form.

Also if anyone can suggest a better place to ask this question, I'd appreciate the tip.