There are several specs that describe the expected behavior. First, the CSS Box Alignment specification states the following: 

https://drafts.csswg.org/css-grid/#grid-baselines

"3- Otherwise, the grid container has no first (last) baseline set, and one is synthesized if needed according to the rules of its alignment context." 

In the same spec, a synthesized baseline is defined as follows:

https://drafts.csswg.org/css-align-3/#synthesize-baseline

"To synthesize baselines from a rectangle (or two parallel lines), synthesize the alphabetic baseline from the line-under line, and the central baseline by averaging the positions of the two edges or lines. See CSS Inline Layout 3 § Synthesizing Alignment Metrics for rules on synthesizing additional baselines.

Note: The edges used to synthesize baselines from a box depend on their formatting context: inline-level boxes synthesize from their margin edges [CSS-INLINE-3], table cells synthesize from their content edges [CSS2], and grid and flex items synthesize from their border edges [CSS-GRID-1] [CSS-FLEXBOX-1]."

As it's stated in the box, an inline-level box like the one used in the example, should use its margin edges.

Additionally, the css-inline-3 spec still refers to the current text included in the CSS Writing Mode specification:

https://www.w3.org/TR/css-inline-3/#baseline-synthesis-replaced

 ISSUE 28  Copy over text from CSS Writing Modes and expand for additional baseline values.

https://www.w3.org/TR/css-writing-modes-3/#replaced-baselines

If an atomic inline (such as an inline-block, inline-table, or replaced inline element) does not have a baseline, then the UA synthesizes a baseline table thus:

alphabetic
    The alphabetic baseline is assumed to be at the under margin edge. 
central
    The central baseline is assumed to be halfway between the under and over margin edges of the box. 

The same issue applies to Flexbox; attached the corresponding test case.

Deleted: baseline-flexbox-issue.html 385 bytes

baseline-flexbox-issue.html 385 bytes View Download