"css-position-3: 2.1. Containing Blocks of Positioned Boxes" says:

If the ancestor is an inline box, the containing block is formed by the block-start and inline-start content edges of the first box fragment of the ancestor, and the block-end and inline-end content edges of the last box fragment of the ancestor.

When taking into account bidi (see "css-writing-modes-3: 2.4.5. Reordering-induced Box Fragmentation") it is unclear whether first and last are meant logically or visually.

While we consider visual the only useful and expected interpretation in this case, we normally would assume logical when nothing is specified.

Chrome and Firefox have different interpretations:

Source:

<p dir="rtl"><span style="position: relative;">abc&#x5d0;def<span style="position: absolute; left: 0; top: 0; bottom: 0; border-left: 0.1em solid red"></span></span></p>

Chrome (visual):

Firefox (logical):

This also demonstrates how unexpected the logical version is.

Whether extending the paragraph is enough or the processes requires more detailed specification we are not sure.