They are stylistic choices of the different fonts as well as standards selected by the authorities. Taiwanese fonts usually exhibit 八 and HK/CN usually use 丷 instead
This is the Wikipedia article describing the Taiwanese standardization.
This is the official document by Ministry of Education ROC, explaining the the standardization.
The entire document is summerized in this PDF, but it's quite broken in my viewer: http://22.214.171.124/~moodle/pluginfile.php/339/mod_resource/content/3/%E6%A8%99%E6%BA%96%E5%AD%97%E9%AB%94-%E6%B2%BF%E9%9D%A9.pdf
Hong Kong opted for 丷 as described in this document: http://input.foruto.com/ccc/gongbiu/dzijingbiu/doc/dzijingbiu_ji-pin.pdf of the 常用字字形表, explained in this Wikipedia article. This standardization is followed by the Hong Kong Education Bureau.
All other documents are listed under: http://input.foruto.com/ccc/gongbiu/dzijingbiu/index.htm
In this excerpt, the left is the standard, whereas the right is non-standard.
Regarding the conclusive answer, if your font shows 八 in 兌/关, it is most likely a font following the Taiwanese standard. The Taiwanese government assisted the development of the first Chinese translated Windows, so the Microsoft fonts PMingLiu/KaiTi are mostly following Taiwanese standards.
The image is from: https://www.zdic.net/hans/%E5%85%B3, a site that documents characters. From left to right [Mainland China, Taiwan, Hong Kong] (They used to also document characters used in Korean Hanja and Japanese Kanji). Also notice the dian (點) instead of na (捺) for the last stroke, used in the Taiwanese version
However, if a font isn't a Taiwanese font, it isn't limited to being a PRC font. In fact, Chinese characters are used in other East Asian countries/regions: Vietnam, Japan, Korea, Hong Kong, Macau, Singapore, etc.
And being in a certain physical region does not equate to a local font is used. Take the public transit service in Hong Kong, MTR, for an example, their Ming (serif) fonts are well known to be Japanese fonts, as indicated by the use of a vertical stroke (Japanese) in 亠 instead of a dian (HK).
As for the horizontal stroke in 八, it is most likely a stylistic decision of the font as I couldn't find relevant information online.