When an arc is described using a missing whitespace, which is perfectly valid SVG syntax, it will not be rendered correctly in Illustrator. This was discovered with SVG files compressed by SVGO 1.3+ and later which has the arc path optimisation by default.

This syntax should work as it is a standard part of the SVG specification and is rendered correctly by all major web browsers such as Chrome, Firefox, Safari.

In the attached example an arc is written as:

a.65.65 0 011.109-.459

The string 011.109 contains 3 values:

0
1 (a boolean is required here, it cannot be 11.109)
1.109

This syntax (with missing whitespaces) follows the SVG 1.1 standard: https://www.w3.org/TR/SVG11/paths.html#PathDataBNF

SVGO discussion: https://github.com/svg/svgo/issues/1137