U.S. solar-panel makers First Solar (FSLR) - Get Report and SunPower (SPWR) - Get Report  briefly popped Monday, but later turned negative even though after the Trump administration eliminated an exemption that had allowed some foreign-made solar panels to avoid a 25% tariff.

The U.S. Trade Representative announced Friday that the administration will soon eliminate an exemption that had shielded so-called "bifacial" solar panels from the import duties. Bifacial solar panel have solar cells on both their top and bottom, whereas traditional solar panels have electricity-producing cells on only one side. The devices had enjoyed exemption from a 2018 tariff on many imported solar panels, but the administration plans to end that loophole on Oct. 28.

The news sent First Solar flaring up as much 6% and SunPower 10.4% higher shortly after the market opened, but the gains later evaporated. FSLR was down 1% at $58.71 shortly after 10:30 a.m. ET, while SPWR was 0.7% lower at $10.47.

Cowen and Co. published a note Monday saying that First Solar will be the biggest beneficiary from the rule change, with SunPower seeing benefits to a lesser extent. However, Bloomberg reported that bifacial panels accounted for just 3% of the solar market last year, so it's unclear just how much of a boost U.S. solar companies will actually get from the new rules.