In principle, the same applies to working hours in the home office as to working hours in the office. The employer can decide from when to when you work and in which time window you should take your lunch break - but they do not have to specify this. They also do not have to specify whether it is a maximum break of one or two hours. You have the same entitlement to breaks and rest periods when working from home, but the general rules of the Working Hours Act also apply: a break is mandatory after six hours at the latest (at least 30 minutes), and there must be at least eleven hours between the end of the working day and the next start of work. If, for example, a work email is sent during this time, the rest period starts again. There are exceptions in certain industries or through regulations in the collective labour agreement. The employer cannot dictate what you do during your lunch break.
Anyone who spends (half) a day on a racing bike instead of working from home without prior agreement and therefore works significantly less than agreed is committing a serious breach of duty. Even in the first case, this can lead to dismissal without notice. However, working time fraud to the detriment of the employer must be proven. It is often proven by a detective - the Strava account can also provide evidence.
An accident on the lunchtime training ride is not a work or commuting accident. This also applies to a lunch ride with colleagues. If the company has a time recording system, it is mandatory to clock out. As with all leisure rides, private accident insurance is worth considering.
No law stipulates the most important thing: clear agreements with superiors and colleagues. Then you won't stumble across any legal pitfalls, nor will there be any misunderstandings if other employees in the team have to take over work in the meantime.

Editor