That's not how Bills of Attainder work. They are primarily about punishments without findings of guilt. Particularly, denial of civil rights without trial. Whatever the merits of this case, removing a tax break is not a criminal finding, and having an organization paying the same taxes as most others is not a denial of civil rights.
"The two main criteria that the courts use to determine whether legislation is a bill of attainder are (1) whether “specific” individuals, groups, or entities are affected by the statute, and (2) whether the legislation inflicts a “punishment” on those individuals. The U.S. Supreme Court has also identified three types of legislation that would fulfill the “punishment” prong of the test: (1) where the burden is such as has “traditionally” been found to be punitive (historical test); (2) where the type and severity of burdens imposed are the “functional equivalent” of punishment because they cannot reasonably be said to further “non-punitive legislative purposes” (functional test); and (3) where the legislative record evinces a “congressional intent to punish (motivational test).”
...
"It would appear that the identification of papers and recordings under the control of a named person (the former President) would meet the per se requirement. The Court in Nixon, however, found that the statute was constitutional despite this specificity. In Nixon, the Court found that the bill failed the second prong (punishment) of the test for a bill of attainder, since the act fulfilled
the valid regulatory purpose of preserving information which was needed to prosecute Watergaterelated crimes and was of historical interest.31 As part of this analysis, however, the Court even questioned whether the statute in question met the specificity prong of the two-part test, finding that naming an individual could be “fairly and rationally understood” as designating a “legitimate class of one.”32 Thus, it has been suggested that Nixon stands for the proposition that any level of specificity is acceptable, even the naming of individuals, as long as a rational, non-punitive basis for the legislation can be established."
https://sgp.fas.org/crs/misc/R40826.pdf
Property taxes are not traditionally punishments. The type and severity of the property taxes in this case are entirely in line with taxes that most everyone else pays, and the purpose here entire furthers non-punitive legislative purposes.
Even the single class-member herewould like meet the "rational, non-punitive" test.
There are also a mountain of very carefully crafted tax breaks and subsidies designed to benefit the one particular corporation that qualifies.
I have no idea if the above proposal is a good idea or a bad one, but I would be very surprised if it qualified as a Bill of Attainder.