When unicode-bidi:-moz-plaintext was originally implemented, the CSS Writing Modes Level 3 spec said that the plaintext value was not supposed to have any effect on inline elements. However, that changed some time ago. It currently states as follows:
This value behaves as ‘isolate’ except that for the purposes of the Unicode bidirectional algorithm, the base directionality of each bidi paragraph immediately contained by the element is determined not by the element's computed ‘direction’ as usual, but by following the heuristic in rules P2 and P3 of the Unicode bidirectional algorithm. An element immediately contains a bidi paragraph if the element itself, but none of its descendants, both contains the entire bidi paragraph and is either a block container or a bidi-isolating inline.
In other words, plaintext is supposed to bidi-isolate inline elements just like unicode-bidi:isolate, while giving each bidi paragraph bounded by the element its own base direction by the usual plaintext algorithm (just like it does for non-inline elements).
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Test case 1
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The test as reftest
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The right diff this time
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Test case 2
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Test case 3
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Test case 4
BTW, plaintext is only supposed to affect alignment when it is on a containing block.
I've added a note in: