This diphthong may have appeared only in the Ossiriandic dialect. First, primitive [ǣ] became [ea] in Ossiriandic, though in the Comparative Tables it was marked as a long diphthong ēa (PE19/25). Furthermore, it seems that [a] became [ea] before clusters of consonants beginning with [r] or [l], as with ealc < ᴹ✶alkwā (Ety/ÁLAK). This change is the same as the vowel-breaking process in the Old English language that inspired Ossiriandic, so it may have been limited to Ossiriandic as well.
Reference ✧ PE19/25 ✧ ēa
||✧ PE19/25 (†ǣ > ēa)|