It should help to see older 990s to see how the income for this rental activity compares to dues, premiums, and member-related income. Look at the business meeting schedule as compared to outsider events, for a few years of these activities, and can that be applied for how long they owned the property as a general experience? Is that CPA making a blanket 50% for gain or non-program activity-related as 50% or on a space ratio or what? You state they had meetings there, perhaps it was rented 6 times a year max, but otherwise used by member and organizational activities 95%. The rental income reporting should help reveal this, and were any rental expense reported, or none of the rental activity was reported?

Significance requires quantification.

Under 501(c)(8), this is in essence a pooled insurance and benefits program. I found you these two resources that look helpful:

https://www.irs.gov/pub/irs-tege/eotopicf04.pdf

https://www.reddit.com/r/freemasonry/comments/cfhvbe/looking_for_answers_on_501c8_fraternal_society/...