Having observed precisely the same phenomenon in all manuscripts (written in Oriya script) we collated for our edition of Śrīdhara's Vihāhādikarmapañjikā, Dr. Shilpa Sumant and myself decided to edit with the spelling निरृति. See for example line 9 on the attached extract from p. 33 of our book published last year. At least in Orissa, where our text was composed, the explanation must be connected with the fact that the word would be pronounced nirruti, in other words (the Oriya equivalent of) रृ is perceived as equivalent to र्रु. I would guess a similar explanation will apply in the sources you are looking at.

From: INDOLOGY <indology-bounces@list.indology.info> on behalf of Krishnaprasad G via INDOLOGY <indology@list.indology.info>
Sent: Saturday, June 22, 2019 4:05 AM
To: Indology; bvparishat@googlegroups.com
Subject: Re: [INDOLOGY] A question about writing निर्ॠति or निरृति in Devanagari

I have also found interesting vereses while research

अचं दृष्ट्वाऽधो याति हशश्चोपरि गच्छति।

अवसाने विसर्गः स्यात् रेफस्य त्रिविधा गतिः॥

तुम्बिका तृणकाष्ठञ्च तैलं जलमुपागतम्।

स्वभावादूर्ध्वमायाति रेफस्यैतादृशी गतिः॥

On Sat, Jun 22, 2019 at 9:17 AM Krishnaprasad G <krishnaprasadah.g@gmail.com> wrote:

Dear all

I was doing a small research wchich is the correct method to write the vowel ऋ preceeded by र् consonant. I have checked in the old manuscripts where I could only see निरृति and never निर्ॠति. The later is widely used in modern days and in all the fonts. But I don't understand how can we have conjunct to a vowel. After consulting n number of MSS, consequently, I strongly believe the former is correct. However if any scholar would recomend any other options I would be grateful to know. Thanks

And here is the difference I have made in Rgveda