The augment t is added to a long vowel, when followed by cha in a continuous text.,
Thus ह्रीच्छति, म्लेच्छति, अपचाच्छायते, विचाच्छायते ॥ The augment belongs to the long vowel and not to the whole syllable ending in that long vowel.,
दीर्घात्
Adhyāya 6 · Pāda 1 · Rule 75