S. R. Faruqi:

The word ṣafīr is feminine; the refrain (which is masculine) has a fine piquancy with it. In the verse the subtlety is that the rose-season is a thing to see, but it's been given the simile of the Nightingale's voice, which is a thing to hear. Between goyā and ṣafīr is the pleasure of a zila. An additional pleasure is that he's kept only hearing mention of the rose-season, it wasn't vouchsafed to him to see it-- the way it's not necessary to see the Nightingale as well as hear it. That is, for the speaker the rose-season was limited to the Nightingale's voice.

If in the first line us ne would be considered to have been omitted, then the meaning emerges that 'that one' (that is, the beloved) wasn't even faithful to a single glance, as if she was the rose-season or the Nightingale's voice, that's here today and gone tomorrow. Mir used exactly this imagery in other places, but not with this excellence. From the second divan [{916,9}]:

der rahne kī jā nahīñ yih chaman
bū-e gul ho ṣafīr-e bulbul ho

[it's not a place to stay in long, this garden
whether it be the scent of the rose, or the voice of the Nightingale]

From the fourth divan [{1529,4}]:

bū-e gul yā navā-e bulbul thī
ʿumr afsos kyā shitāb gaʾī

[it was the scent of the rose, or the voice of the Nightingale
the lifetime, alas-- how quickly it went!]

Mir has taken this theme from Mirza Razi 'Danish' [in Persian]:

'The springtime of gatherings and the turmoil of youth
were the voice of the Nightingale and the scent of the rose.'

FWP:

SETS
MOTIFS == GAZE; SPRINGTIME
NAMES == NIGHTINGALE
TERMS

The back-and-forth-ness (not exactly a synesthesia) between hearing and seeing, with the common link of beauty and brevity, feels somehow hypnotic. And the presence of the beloved flickers in and out of the verse, depending on how we read the grammar of the first line. For after all, the structure of the verse is, 'A(?) did not do B, as if / C, D was'. There are various ways to hook up such flexible relationships. (Although the fact that mausam is masculine and ṣafīr is feminine and the verb is in the past tense does help to limit the possibilities.)

How does one keep faith 'to' ('with'? 'to the extent of'?) a single glance or gaze? That one didn't offer a single glance? That one didn't stay as long as a single glance? That one ignored the lover's imploring gaze? Despite all explanations, the idea remains provocative and mysterious.

SRF treats ṣafīr as a synonym for 'voice' [āvāz], but in fact the word has a special reference to the 'singing of a bird' (see the definition above) that's particularly appropriate to the verse. Then, goyā of course literally means 'speaking, speaker'.