Wiktionary explains the two glyph origins of 「兹」 as the following:
Unorthodox variant simplified from 茲 with 䒑 as the top component instead of 艹 found in the Ming dynasty orthographic dictionary 《俗書刊誤》.
Variant form of 玆 with 䒑 instead of doubled 亠 as the top component. Found in the Tang dynasty orthographic dictionary Ganlu Zishu 《干祿字書》, which lists both 兹 and 茲 as unorthodox forms of 玆.
For characters in the phonetic series 「兹」, most sources (from my experience) explain the characters as semantic 「X」 + phonetic 「茲」. To illustrate, here is a part of the entry for 「滋」 from Chinese Linguipedia:
And below is the entry for 「滋」 in 說文解字:
However, it seems that there are two seal script forms; one that matches the structure 「水」 + 「茲」 (left), and the other matches the structure 「水」 + 「玆」 (right).
Compare the seal script forms of 「茲」 and 「玆」 respectively:
It also appears that abbreviated 「絲」 was the original phonetic component in older forms like oracle bone script, and was later replaced with 「茲」 or 「玆」. Both of these characters have the Fanqie spelling of 子之切, but seem to have different origins.
As a phonetic component, were both 「茲」 and 「玆」 used, and later converged into 「兹」? Or is there a different story behind it?