Слушайте, вообще-то у Вас все должно быть хорошо…
9 FAM 502.1-1(D)(7) Retrogression of Application Final Action Dates
(CT:VISA-902; 07-29-2019)
a. In order to seek to acquire lawful permanent residence, an alien beneficiary must actually have one full year of visa availability. If an Application Final Action Date retrogresses (e.g., employment-based third preference numbers are unavailable) or the preference category changes (e.g., F1 converts to F3) within one year of visa availability and the visa applicant has not yet sought to acquire LPR status, then once a visa number becomes available again the one year period starts over. The alien beneficiary’s age under the CSPA is re-determined using the subsequent Application Final Action Date.
b. If a visa availability date retrogresses before the visa had been available for one full year, any actions taken within one year of the visa becoming available and that satisfy the “sought to acquire” requirement (see 9 FAM 502.1-1(D)(6)) above will be sufficient to lock in the applicant’s CSPA age as of the first day the visa became available during this time period.
EXAMPLE: If a visa became available on June 1, 2015, the visa availability date retrogressed on July 1, 2015 and the applicant sought to acquire a visa on August 1, 2015 by paying IV fees, the applicant’s CSPA age would be locked in as of June 1, 2015 based on seeking to acquire within one year of visa availability. The next time the visa becomes available, the consular officer would calculate the CSPA age as of June 1, 2015.
Поскольку у Вас все условия п. b) выполнены - виза доступна 6 июля, потом 1 августа стала недоступна, в течение года Вы выполнили все условия (оплатили визовый сбор и подали DS-260), а потом в некий момент виза стала доступна (1 декабря), то они должны зафиксировать возраст по состоянию на 6 июля (точно, как в их примере).
Вот Вам ссылка на полный FAM:
https://fam.state.gov/fam/09FAM/09FAM050201.html#M502_1_1_D_6