Let me redescribe this issue with some background knowledges.

Solus 4.1 comes without complete support of CJK font fallbacks unless additional font packages are introduced. In this case, we use noto-CJK.
noto-CJK brings support of 5 standards of kanji ideograph defining how strokes are written.
These standards are used in Mainland China, Taipengjinma (Taiwan + Penghu + Jinmen + Mazu), Hongkong, Japan, and Korea.

Regardless the system UI language settings, a web browser at this age is supposed to:

1. read the lang tag from the HTML file:

zh-Hans [Region Neutral] [Simplified Chinese]
zh-Hans-CN [Used in Mainland China] [Simplified Chinese]
zh-Hans-HK [Used in HK] [Simplified Chinese]
zh-Hans-MO [Used in Macau] [Simplified Chinese]
zh-Hans-SG [Used in Singapore] [Simplified Chinese]
zh-Hans-TW [Used in Taiwan, Penghu, Jinmen, Mazu] [Simplified Chinese]
zh-Hant [Region Neutral] [Traiditional Chinese]
zh-Hant-CN [Used in Mainland China] [Traiditional Chinese]
zh-Hant-HK [Used in HK] [Traiditional Chinese]
zh-Hant-MO [Used in Macau] [Traiditional Chinese]
zh-Hant-SG [Used in Singapore] [Traiditional Chinese]
zh-Hant-TW [Used in Taiwan, Penghu, Jinmen, Mazu] [Traiditional Chinese]
ja [Region Neutral] [Japanese]
ja-JP [Used in Japan] [Japanese]
ko [Region Neutral] [Korean]
ko-KR [Used in South Korea] [Korean]

(Note that "zh-TW / zh-CN / zh-HK" are supposed to be deprecated but I found that they are still being used in most Linux distros. This confuses me well.)

2. According to which tag the browser read from the HTML page (it can be defined in a span, div or the entire HTML block), the browser is supposed to ask the operating system (or Chrome Advanced Font Settings / FireFox per-language font settings) to see which CJK fallback font is suitable for this lang tag, and use this font to display the content.

To mention: the full-width period symbol 。 is shown in the middle if displayed through zh-Hant-TW fonts (if they are made according to local educational standards) like "Noto Sans CJK TC".

Now let's repeat this issue:

1. Let Solus 4.1 system UI language to be set as any CJK language (except zh-TW Chinese Traditional Taiwan). // Recommending zh(-Hans)-CN or Japanese or Korean.

Then, reboot your system.

2. Open either Chrome or Firefox. Make sure their font settings are vanilla or set as "sans-serif" for all languages. If Chrome Advanced Font Settings plugin is installed, uninstall it.

3. Open any page from Chinese Wikipedia (zh-TW variant) because its HTML lang tag is "zh-Hant-TW". In this case we use https://zh.wikipedia.org/wiki/Budgie_(%E6%A1%8C%E9%9D%A2%E7%8E%AF%E5%A2%83)

Expected Result: The full-width period symbol 。 is expected to be shown both vertically and horizontally middle in its em square box, proving that the Noto Sans CJK TC font installed in the current Solus 4.1 system is correctly read and used in this webpage.
Actual Result: Even if using browser inspector to change "zh-Hant-TW" to its deprecated form "zh-TW", the issue still exists that only the CJK fallback font of the current Solus 4.1 UI lanuguage settings gets used in this page.

To mention that macOS 10.15 doesn't have this issue (I didn't test previous macOS builds).
Since both Chrome and Firefox on Solus 4.1 have this issue, I believe that there must be some prime cause in the operating system.