[RPG] Are a polymorphed beast’s attacks magical

No, the polymorphed creature can only use actions that a normal creature with the new form would be able to do. It comes directly from the definition of “game statistics”. The Statistics section of the The Monsters Manual starts with:

A monster's statistics, sometimes referred to as its stat block, provide the essential information that you need to run the monster.

The subsections include not only static numbers “Ability Scores” and “Speed” but also “Actions” meaning that RAW actions are also stats (and I cannot name a reason why it wouldn't be RAI as well). Under “Actions” it is written:

When a monster takes its action, it can choose from the options in the Actions section of its stat block or use one of the actions available to all creatures, such as the Dash or Hide action, as described in the Player's Handbook.

Thus the polymorphed character's actions are limited to those the creature would be able to do were it a normal creature.

As for the possible inconsistency with the concentration ("A Wizard can polymorph self, therefore polymorph doesn't break concentration. If concentration is a class feature and it survives polymorph, all other class features can be used too.") there are two choices here:

1. Concentration is not something everyone has automatically. E.g. if a creature would not be normally able to cast spells it cannot concentrate on them either. In this case there is no consistent RAW answer: "statistics" cover all the aspects of creature -> if the creature cannot concentrate, polymorph breaks concentration -> polymorph cannot be used at self, even though PHB clearly states that it was intended to be used this way. Also Sage Advice says that concentration is not broken by polymorph, so this is definitely not a path to go with.

   You can’t cast spells while you’re transformed by polymorph, but nothing in the spell prevents you from concentrating on a spell that you previously cast before being transformed.

2. Concentration is something everyone can do and/or let's use common sense. The point of the offensive use of Polymorph is to render a creature useless in battle, i.e. if a creature is turned into a sheep it is as harmless as one. If the class features were preserved we would have sheep going into rage (barbarian), teleporting (shadow monk) or... turning into dinosaurs (unless druids are considered to be shapechangers and thus are automatically saved) which is bizarre. Therefore, all the class/race features are gone. As for the possible inconsistency with concentration, it comes from a simple fact that designers cannot see all the connections between all the bits of the rules and sometimes we can only rely on how we think things are supposed to work.

The mace provides a magical attack that does bludgeoning damage.

The term "magical attack" is defined as:

... a magical attack is an attack delivered by a spell, a magic item, or another magical source (Basic Rules, DM, p. 4; under Vulnerabilities, Resistances, and Immunities)

Let's also clear up the misconception exposed by the wording of the question. There is no such thing as "magical damage". The list of damage types includes "radiant", "necrotic", "bludgeoning","fire", and so on. The +1 Mace does Bludgeoning damage, but it is a magic item and therefore does so as a magical attack. Hence it bypasses the resistance to "bludgeoning damage from non-magical attacks".

The mace does not bypass resistance to bludgeoning attacks in general. There are not many examples, but the Treant is resistant to Bludgeoning damage, period, and is therefore resistant to this mace. (The ettercap's web is also immune to bludgeoning damage, and there may be some other examples along those lines.)